Reuse bootstrap: --runner WrapperRunner

Related to https://github.com/brianium/paratest/issues/26
Results are the same with the old runner (Runner) and new one (WrapperRunner) afaik. Workers that crash trigger an exception in the main process. I plan to do more work on this (extracting a WorkerPool object, restarting workers and signaling crashed tests as errors, investigating performance), but this is the minimum marketable feature.